A compressor (compression machine) used in a supercharger such as a turbocharger of an automobile includes a compressor housing configured to house an impeller that includes a plurality of blades.
The compressor housing includes an intake port that takes in air toward the impeller, a discharge scroll chamber that is formed in a circumferential direction at an outer circumference of the impeller and guides air discharged from the impeller to outside, and other than the above, a portion to house the impeller, a diffuser section and the like.
As a method of manufacturing a compressor housing, for example, there is a method to form the same by gravity casting. In this case, since the casting can be performed by using a so-called core, the degree of freedom for shape formation is high to cope with a complicated shape. However, productivity is low due to a long casting cycle, and also, cost therefor is high.
On the other hand, there is a method to form a compressor housing by die casting. In this case, compared to the gravity casting, the productivity is satisfactory and the cost therefor is low due to short casting cycle. However, the method is available only when it is designed to be formed by die cutting. The degree of freedom for shape formation is low, thus failing to cope with a complicated shape. Due to this, even if the shape can be formed by the gravity casting, there may be cases in which such shape (especially, shapes of wall surfaces of a discharge scroll chamber that may influence a compressor performance) cannot be reproduced by the die casting.
Thus, as shown in FIG. 7, a technique that configures a compressor housing 91 by dividing the same into two components, namely a scroll piece 92 and a shroud piece 93, forms these components by die casting, and assembles the same has been proposed (see patent document 1). In this compressor housing 91, a wall surface forming section 962 that forms a part of an outer circumferential side wall surface of a discharge scroll chamber 912 (outer circumferential wall surface 913) is provided in a back plate 961 opposing a diffuser surface 931 of the shroud piece 93, whereby the wall surface of the discharge scroll chamber 912 is formed of the scroll piece 92, the shroud piece 93, and this wall surface forming section 962.